FIGS. 1(A) and 1(B) show the principle of a magneto-optic recording method in which a magneto-optic overwritable recording medium is used. Referring to FIG. 1(A), a magneto-optic overwritable medium 10 is configured such that a magnetic layer embodying a memory layer 12 is formed on the main surface of a disc-shaped substrate 11 made of a glass or a polycarbonate, and such that a magnetic layer embodying a recording layer 13 is formed on the memory layer 12, the memory layer 12 and the recording layer 13 being coupled by exchange interaction. While the overwritable medium 10 is made to rotate at several thousand rpm, a laser light beam is applied to the medium through the substrate 11 thereof. A bias magnet 15 is provided opposite a part of the recording layer 13 to which part the laser light beam is applied. An initializing magnet 16 is provided in front of the bias magnet 15 so that the two magnets are separated at a distance along the circumferential direction of the medium. The bias magnet 15 and the initializing magnet 16 face the recording layer 13 in opposite directions in terms of polarity.
Temperature--coercive force characteristics of the memory layer 12 and of the recording layer 13 are as indicated by L.sub.12 and L.sub.13 of FIG. 1(B), respectively. A Curie point. TC.sub.R of the recording layer 13 is prescribed to be higher than a Curie point TC.sub.M of the memory layer 12. A broken line indicates the intensity of the bias magnetic field of the bias magnet 15, and an alternate long and short dash line indicates the intensity of the initial magnetic field of the initializing magnet 16.
In such a magnetic medium 10, the recording layer 13 is magnetized, at a room temperature, in a direction specified by the initializing magnet 16. Accordingly, information recorded in the recording layer 13 as variations in magnetization is erased by means of the magnet 16. When a laser light beam 14 having a large power (P.sub.H) is applied to the memory layer 12 so as to heat it beyond a temperature TH, a subsequent cooling process ensures that the directions of magnetization of the recording layer 13 and the memory layer 12 are aligned with the magnetization direction of the bias magnet 15, so that record marks are formed. When the power of the applied laser light beam 14 is low (P.sub.L), and the memory layer 12 is heated to a temperature within a range between TL and TH, the recording layer 13 is not magnetized by the bias magnet 15, and the magnetization direction of the memory layer 12 is aligned with the magnetization direction of the recording layer 13 (the magnetization direction of the initial magnetizing magnet), due to exchange interaction. In such an information recording process, an overwrite process not involving an erasing process can be carried out on the magneto-optic overwritable medium 10.
In a magneto-optic recording process, information modulated through (2, 7) RLL coding (run length limited coding) is generally recorded, an example of how this modulation is effected being shown in FIG. 2(A). In (2, 7) RLL coding, it is stipulated that two channel bits "1" be separated by a sequence of between two and seven channel bits "0". That is, the conventional recording is performed such that the light beam power P.sub.H is used, as shown in FIG. 2(B), to correspond to a state "1" of recorded data shown in FIG. 2(A) and such that light beam power P.sub.L is used to correspond to "0". This way, a pit position recording can be effected in which record marks as shown in FIG. 2(C) are formed. A restored waveform as shown in FIG. 2(D) is obtained when the information is read. Referring to FIG. 2(C), hatched areas indicate record marks where there are reversions in magnetization direction. With such a (2, 7) RLL coding, there is no fear of synchronization being lost during a read operation, since record marks are formed at a maximum of every seventh pit however long a sequence of the same logical value is in the recorded data. At the time of reading the data, a light beam power P.sub.R smaller than the above P.sub.L is used.
The recording of the magneto-optic overwritable medium is effected on the basis of the heating of the medium by means of the laser light beam. When a 1.5 pattern, corresponding to the recorded data "1001" where the marks are closest to each other, is performed, a large amount of heat used for recording the marks is transferred to non-record areas between the marks. Hence, there may arise a case where the marks are very close to each other, as indicated by broken lines in FIG. 2(C) and 2(D), or a case where they overlap each other. .tau. indicates a length of an information bit cell before being subjected to (2, 7) RLL code modulation. Solid lines in FIG. 3 indicate a restored signal carrier--noise ratio (CN ratio) obtained as the light beam powers P.sub.L and P.sub.H used at the time of recording the above 1.5.tau. pattern are varied. In this pattern, the CN ratio is at a maximum level when the light beam power P.sub.L is small. Contrastingly, when a 4.tau. pattern, corresponding to the recorded data "100000001" where the marks are widest apart from each other, is recorded, a non-mark area extends over a large length, and the heat that produced a mark exercises relatively small influence on the mark adjacent to it. Hence, the CN ratio of the restored signal is as indicated by broken lines in FIG. 3. It is found that a large light beam power P.sub.L is required to obtain a sufficient-CN ratio for a given light beam power P.sub.H. P.sub.R indicates a light beam power used in a read mode. In this way, the conventional overwrite magneto-optic recording method has a disadvantage in that the optimal light beam power, particularly the light beam power P.sub.L, for maximizing the CN ratio depends on the recorded data pattern, and in that, consequently, an optimal combination of the light beam powers P.sub.L and P.sub.H capable of restoring the data at a high CN ratio, irrespective of the recorded data pattern, cannot be determined.